The present invention provides a synchronous frequency encoding technique (SFET) for circuit emulation of real-time circuit-switched isochronous telecommunication services in a broadband asynchronous transfer mode (ATM) network. More particularly, the invention provides for recovery of a source node service clock frequency at the destination node in a synchronous network where the source and destination nodes are controlled by timing signals derived from a single master clock.
It is essential to the proper delivery of isochronous service traffic in a broadband network that the clock controlling the destination node buffer be operating at a frequency precisely matched to that of the service signal input at the source node in order to avoid loss of information due to buffer over-or under-flow. However, unlike the circuit-switched transport of service data wherein the clock frequency at the destination node may be traced directly back to that of the source node by the regular, periodic arrival of the isochronous traffic, transport in an ATM network inherently results in cell jitter, i.e. the random delay and aperiodic arrival of cells at a destination node, which essentially destroys the value of cell arrival instances as a means for directly recovering the original service signal input frequency.
Such cell jitter, generally the result of the multiplexing of transport cells in the broadband network and the cell queuing delays incurred at the ATM switches in the network, is substantially unpredictable. Thus, little is known of the statistics of cell arrival time beyond the fact that the average cell delay is a constant, assuming that the ATM network provides sufficient bandwidth to ensure against loss of cells within the network. As a means for closely approximating service signal frequency at the destination node, some consideration had previously been given to utilizing a direct extension of circuit-switched timing recovery practices which rely entirely upon a buffer fill signal as the basis for recovery of the source timing. However, due to the lack of knowledge of statistics of the cell jitter, this approach would have required a phase-locked loop with very low cut-off frequency (in the order of a few Hz) and would thus have resulted in excessive converging time and degradation of jitter and wander performance.
A number of schemes have more recently been proposed to improve upon such a conventional manner of recovering service timing in the presence of cell jitter, yet none has achieved this end economically and without extensive control systems of notable complexity. Singh et al., for example, in "Adaptive Clock Synchronization Schemes For Real-Time Traffic In Broadband Packet Networks," 8th European Conference on Electrotechnics, Stockholm, Sweden, June 1988, and "Jitter And Clock Recovery For Periodic Traffic In Broadband Packet Networks," IEEE Globecom '88, Florida, December 1988, have proposed algorithms which attempt to more closely estimate cell jitter statistics and derive timing recovery from those indications. These recursive approaches, suggested to be applicable to both synchronous and non-synchronous networks, rely upon the interaction of increasingly complex algorithms which would require the noted extensive controls for implementation.
Common to these prior service clock recovery proposals is the element of estimating the statistics of cell jitter in order to establish some basis for subsequent destination node clock control. Thus, whether applied in a synchronous network (identical source and destination node timing frequencies derived from the same clock) or a non-synchronous network (nominally identical source and destination timing frequencies derived from different clocks), any such practice continues to suffer from the uncontrollable and unpredictable variants which are the cause of cell jitter. The present invention, on the other hand, albeit primarily applicable in a synchronous network, eliminates entirely the factor of cell jitter and thereby reduces the problem of clock recovery to the simpler and readily accomplished one of resolving the substantially constant difference in frequency between the isochronous service clock and the network control clock.